1. Field of the Invention
The present invention generally relates to a thin film transistor (TFT) array substrate and a repair method for a TFT array substrate, and more particularly, to an auxiliary capacitor structure of a TFT array substrate used in display apparatuses and a method for repairing point defect produced during the fabrication process of the TFT array substrate.
2. Description of the Related Art
In recent years and continuing, cost reduction and improved image quality are demanded for display units of information technology equipment. Such demand is especially remarkable for liquid crystal displays (LCDs) used in cellular phones, personal digital assistants, notebook PCs, view finders of video cameras, and the like.
In a liquid crystal display, liquid crystal is sandwiched between a TFT array substrate, in which an array of thin film transistors is formed for driving pixels, and an opposite substrate. Displayed images are controlled by regulating orientation of the liquid crystal. It takes several miliseconds until orientation of the liquid crystal is stabilized, and an electric charge supplied by switching of the thin film transistor has to be maintained during this period. For this reason, an auxiliary capacitor is required to achieve high-quality image display.
Because a defect in the auxiliary capacitor leads to a defect of a pixel, it is important to remove deficiencies in auxiliary capacitors, and an improved and reliable structure of auxiliary capacitors is required.
In reality, millions of TFTs are often formed on a substrate, and it is difficult to manufacture a TFT array substrate with substantially no point defects. Accordingly, technology for repairing a point defect detected by inspection during the manufacturing process of the TFT array substrate becomes important.
FIG. 1 is a plan view of a pixel area of a conventional TFT array substrate, FIG. 2A is a cross-sectional view taken along the C–C′ line of FIG. 1, and FIG. 2B is a cross-sectional view taken along the D–D′ line of FIG. 1. As shown in FIG. 2, a semiconductor film 104 is formed over a gate electrode 101 via a gate insulating layer 103. A region of the semiconductor film 104 that faces the gate electrode 101 functions as a channel region. Over the semiconductor film 104 are formed a source electrode 107 and a drain electrode 106 via an impurity diffusion layer 121. The source electrode 107 and the drain electrode 106 face each other across an etching stopper 105 (See FIG. 1).
On the right-hand side of FIG. 2A, an auxiliary capacitor electrode 102 is formed in the same level as the gate electrode 101. A middle electrode (or an opposite electrode) 109 is positioned over the auxiliary capacitor electrode 102 with the gate insulating layer 103 between them to form an auxiliary capacitor. To be more precise, the middle electrode 109 is formed over the gate insulating layer 103 via the semiconductor film 104 and the impurity diffusion layer 121.
The middle electrode 109 is connected to a pixel electrode 112 through a contact hole 111. The pixel electrode 112 is also connected to the source electrode 107 through another contact hole 111. Accordingly, the source electrode 107 and the middle electrode 109 are electrically connected to each other via the pixel electrode 112 and contact holes 111. With this conventional structure, a conducting pad is required for contact on the source electrode 107, which reduces the aperture ratio.
If, in this TFT array substrate, malfunction or bad electrical contact occurs in a TFT, a voltage is not supplied to the pixel electrode 112, which results in a point defect. To deal with this problem, the point defect is detected by electrically inspecting the capacitance of the auxiliary capacitor during the fabrication process of the TFT array substrate. Upon detection of a point defect, such defect is to be repaired.
Conventionally, extra TFTs and isolated electrodes are provided in advance in the TFT array substrate for repair purposes, expecting occurrence of point defects. If a point defect pixel is detected, that pixel is repaired using the extra TFT and the isolated electrode.
For example, JP 10-161156A discloses a technique for repairing a point defect pixel. With this technique, an isolated electrode is arranged between adjacent pixels, and the pixel electrode of a point defect pixel is connected to that of the adjacent normal pixel via the isolated electrode by laser irradiation.
In general, when a necessary level of voltage is not supplied to a pixel electrode, that pixel becomes a point defect pixel in the normally-black mode. With the technique disclosed in JP 10-161156A, a required potential difference is produced in the point defect pixel by connecting the pixel electrode of the point defect pixel to that of the adjacent normal pixel.
Another technique is disclosed in JP 2002-278476A, which technique does not require extra TFTs or isolated electrodes. With this technique, the malfunctioning transistor (TFT) of the point defect pixel is cut off from the pixel electrode by a laser, and the separated pixel electrode is connected to that of a normally operating adjacent pixel by a metal film formed by laser CVD such that electric connection is obtained. This repair method makes use of the regularity in arrangement of the pixel array.
However, with the conventional repair methods, a voltage sufficient to support two pixels has to be guaranteed in spite of the fact that the voltage and the auxiliary capacitance supplied from the normally operating TFT are for a single pixel. To this end, driving means and driving voltages have to be determined predicting necessity of repair. This leads to increase of power consumption.
Concerning the image quality, the above-described conventional repair methods are acceptable as long as the repaired (or connected) two adjacent pixels belong to an area to be displayed with the same tone. However, if the boundary or the outline of a pattern extends between the repaired two adjacent pixels, and if these two pixels are to be displayed with different tones, then one of the pixels will be recognized as a dark spot or a bright spot.
Along with the structural problem, that is, the reduced aperture ratio, of the conventional TFT array substrate, the problems in the conventional repair methods degrade the image quality of a liquid crystal panel.